Add the egg whites and sugar to a heat proof bowl over a pot of simmering water. Heat while stirring with a whisk for about 5 minutes or until the sugar has dissolved. Feel the mixture between your fingers and it should be smooth.
Pour the egg white and sugar mixture into the bowl or your stand mixer or a large bowl if you are using a hand mixer. Beat on low speed with the whisk attachment until the mixture becomes opaque. Turn the speed up to medium high and beat for about 10 minutes or until the meringue has reached the stiff peak stand and is at room temperature.
With the mixer on low add the butter a tablespoon at a time, waiting until it is fully incorporated before adding the next. Add in the vanilla essence and scrape down the bowl to make sure everything is evenly combined.